This invention relates generally to gas turbine engines, and more particularly, to methods and apparatus for operating gas turbine engines.
Gas turbine engines typically include high and low pressure compressors, a combustor, and at least one turbine. The compressors compress air which is mixed with fuel and channeled to the combustor. The mixture is then ignited for generating hot combustion gases, and the combustion gases are channeled to the turbine which extracts energy from the combustion gases for powering the compressor, as well as producing useful work to propel an aircraft in flight or to power a load, such as an electrical generator.
When engines operate in various conditions, foreign objects may be ingested into the engine. More specifically, various types of foreign objects may be entrained in the inlet of a gas turbine engine, ranging from large birds, such as sea gulls, to hailstones, sand and rain. The foreign objects may impact a blade resulting in a portion of the impacted blade being torn loose from a rotor. Such a condition, known as foreign object damage, may cause the rotor blade to pierce an engine casing resulting in cracks along an exterior surface of the engine casing, and possible injury to nearby personnel. Additionally, the foreign object damage may cause a portion of the engine to bulge or deflect resulting in increased stresses along the entire engine casing.
To facilitate preventing the increased engine stresses and the possible injury to personnel, at least some known engines include a metallic casing shell to facilitate increasing a radial and an axial stiffness of the engine, and to facilitate reducing stresses near the engine casing penetration. However, casing shells are typically fabricated from a metallic material which results in an increased weight of the engine and therefore the airframe.
The high specific strength and stiffness of polymeric composite materials offer attractive weight reduction benefits for jet engine components. The majority of current composite components are fabricated with material in a unidirectional tape or woven fabric form. The required properties needed for a specific component are achieved by assembling many layers of the chosen tape or woven fabric into one monolithic shell. Unfortunately these forms of composite material exhibit poor characteristics regarding crack propagation. Increasing the thickness of the component can reduce the stresses that initiate a crack to overcome this deficiency. However, this solution reduces or can eliminate any weight savings benefit of composite materials.